Google Calendar
Google Calendar has finally been updated. Well, it has for Android devices. It is a significant update that may possibly increase your usage. I don't know many people that actually use the calendar provided by Google, but I am regular (but light) user. I couldn't wait for Google to push the update to me so I downloaded the APK to update my phone.
This is a HUGE update. The best feature will be having Google automatically create events based on Gmail messages. Google Now showed these events on cards when they were about to arrive. While it is great to have the cards easily accessible at those times, it was impossible to view those events on your own since those Google Now cards are generated on their own; this was semi-annoying. Now, you can plan your schedule around these events with ease!
The schedule view and assist will be nice but I'm not sure I'll actually use those features. I have always used the calendar widget that is basically the condensed form of the schedule view. And I have always thought that creating events on Google Calendar was extremely simple already; not sure how this will make the process easier.
A tip for the novice Google Calendar user, there are public calendars you can browse and use, and you can subscribe to friend's calendars (if they let you).
I'm still test driving Google Inbox right now but I'm not seeing much of a difference in usage between it and Gmail (I don't get many critical emails daily). But Gmail has added the capability to have multiple accounts from different services; this may be the reason I go back to the Gmail application.
A tip for Google applications, you don't need to click the top-left menu button to get to the menu. You can simply swipe from the left edge; it makes it easier for one-handed use. From what I can tell, this works on all Google applications on Android (not sure on iOS) except for the Google+ application. It's ironic that the Google+ app does not have this feature since I think it was the first to have it...